Conventionally, with application products, there have been various suggestions for products wherein the color of the material for application either disappears or changes from the instant that they are applied. Here, the “application product” is a general concept that broadly refers to a product used in order to apply a material for application, not limited to glue, an adhesive, a agglutinant, or a colorant such as ink, but oils and other materials as well onto a subject item.
Accordingly, as one of the abovementioned application products, there is a product commonly referred to as glue sticks (for example, refer to Non-Patent Document 1). In this glue stick, a solid glue presenting alkalinity is contained in a colored state inside a sealed housing by being mixed with a pH indicator. Then, with the solid glue in a colored state, after being applied, the color of the abovementioned pH indicator fades due to contact with air, and in other words, the process of decolorizing occurs. In addition to the advantage of a user being able to easily recognize areas where the glue has not been applied since it is possible to clearly see exactly where the glue has been applied immediately after application, such glue also has the advantage of the areas in which the glue was used being unnoticeable since the glue becomes colorless as described earlier, and these advantages have been a large factor in such glue being widely used by users.
Currently, in order to make it possible to further clearly see exactly where the glue has been applied, there is a demand for a product that has further stronger colors while also being capable of decolorizing to be further unnoticeable after application.
Incidentally, photochromic compounds having characteristics wherein the compounds exhibit color when a light of a certain wavelength is irradiated thereon and become colorless when a light of a different certain wavelength is irradiated thereon are known. Many of these photochromic compounds have characteristics wherein the compounds are capable of clearly exhibiting color when an ultraviolet light of a certain wavelength is irradiated thereon and become colorless when a visible light of a certain wavelength is irradiated thereon. Accordingly, as an example of an application product which makes use of these photochromic compounds, there is the example of photochromic compounds being put to use in writing materials such as markers (for example, refer to Patent Document 1).
However, in regard to these photochromic compounds, although compounds having characteristics wherein the compounds become colorless when a light of natural conditions is irradiated thereon are known (for example, refer to Patent Document 2), the spiropyran-based photochromic compound used in said document is a compound that, regardless of being kept in a light-shielded state, is not able to maintain a state of being colored over a long period of time and gradually decolorizes. For this reason, even if the application product is manufactured in a state of being colored, since the decolorizing process occurs during the stages of storage and distribution that follow, it is not possible for the compound to exhibit adequate color.